1. Field of the Invention
The present invention relates to an engine control device and an engine control method for a straddle type vehicle, and more specifically to an engine control device and an engine control method that are responsive to a rider's accelerator operation.
2. Description of Related Art
The amount of intake air to a two-wheeled motor vehicle engine is controlled by adjusting the opening of a throttle valve in response to the displacement of an accelerator grip operated by the rider. In the case in which the accelerator lever and the throttle valve are mechanically linked to each other by a linkage, the relation between the opening of the accelerator lever (accelerator displacement) and the opening of the throttle valve (throttle opening) is linear. However, the relation between the throttle opening and engine torque is not linear. Engine torque characteristics relative to the throttle opening, or accelerator displacement, vary depending on engine speed, which are generally shown in FIG. 7.
More specifically, when the engine speed is low, a maximum peak value of the engine torque is generated in the range in which the accelerator displacement (throttle opening) is small. As the engine speed increases, the accelerator displacement for the maximum peak value becomes larger. It should be noted that linearity of the engine torque relative to the accelerator displacement decreases for lower engine speed. As the accelerator displacement for the maximum peak value is further increased, the range in which the engine torque decreases or the range in which the engine torque does not change can be distinct, which is called an over venturi effect.
To avoid the over venturi effect, there is a method in which a sub throttle valve is provided upstream from the throttle valve to adjust the opening of the sub throttle valve. In this method, when the throttle valve is excessively opened, the sub throttle valve is controlled in the closing direction so that the flow of air passing through an intake passage accelerates and intake efficiency increases. The over venturi effect is thereby avoided. However, this method requires the sub throttle valve, resulting in an increase in size of a throttle body and manufacturing cost.
Such a method as shown in FIGS. 8(a) and 8(b) is, therefore, known to avoid the over venturi effect not by using the sub throttle valve but by merely adjusting the opening of the throttle valve. FIGS. 8(a) and 8(b) are graphs showing the relation between the accelerator displacement and the throttle opening, which correspond to FIGS. 6(a) and 6(b). This method is designed such that the throttle valve is not opened larger than a throttle opening at which the maximum peak value of the engine torque is generated, as shown in FIGS. 8(a) and 8(b). As a result, any decrease in engine torque due to the over venturi effect is avoided for all engine speeds, as shown in FIG. 9.
However, according to this method, the engine torque is constant in the range in which the accelerator displacement is large, and therefore the engine torque does not increase for any increase in accelerator displacement. Thus, no torque control can be executed in such range. The rider, therefore, cannot obtain a feeling of oneness with the vehicle, resulting in poor riding comfort.
To solve the foregoing problems, Japanese Patent Document JP-A-Sho 61-106934 discloses a method in which engine output is increased for an increase in accelerator displacement for the overall range of rider's accelerator operation. As shown in FIG. 10, this method is designed such that target amount-of-intake-air characteristics relative to the accelerator displacement for each engine speed are continuously increased in response to an increase in the accelerator displacement. When the accelerator is fully operated (i.e., at a maximum opening of the accelerator lever), a maximum amount of intake air is obtained. Engine output thereby can be smoothly increased in response to an increase in the accelerator displacement independently of the engine speed, so that a satisfactory acceleration feeling is obtained. The throttle valve is driven and controlled to an opening at which the target amount of intake air shown in FIG. 10 is provided.
According to the method disclosed in JP-A-Sho 61-106934, the engine output can be increased linearly in response to the rider's accelerator operation for the overall range of the accelerator displacement. However, the target amount of intake air is gradually increased in proportion to the accelerator opening, based on the maximum amount of intake air when the accelerator is fully operated. This raises a problem of a poor response of the engine output when the accelerator displacement is small (when the accelerator lever is started to be opened).